Voltage controlled oscillators (VCOs) are the integral part of many communication systems, which use mixers to up-convert or down-convert signals. Mixers may up/down convert signals by employing local oscillators (LOs) to generate LO signals at a corresponding carrier frequency. Many LOs may use one or more oscillators (e.g., VCOs such as CMOS VCOs) to generate the LO signals. CMOS VCO performance metrics have not improved significantly over the last decade. An existing work that reported a VCO Figure of Merit (FOM) introduced a second resonant tank at a source of the differential pair, where the second resonant tank was tuned to twice the LO frequency (FLO).
The additional tank circuit used in the existing work may provide a high common-mode impedance at the frequency of 2×FLO. The high common-mode impedance may prevent the differential pair transistors from conducting in the triode region and, so, can prevent the degradation of the oscillator's quality factor (Q). As a consequence the employed topology can achieve an oscillator noise factor (F) of close to 2, which is near the fundamental limit of a cross-coupled LC CMOS oscillator. A drawback of this solution is the additional area and the routing complexity demanded by an extra inductor of the additional tank circuit.